Contacts are required in solar cells to collect the current produced by the photon induced charge carriers which are separated by a charge collecting barrier, one form of which is known in the art as a p/n junction. Such a p/n junction in silicon solar cell wafers is located within 1 micron from that surface of the solar cell which is exposed to sunlight due to several absorption considerations. The contacts placed on the exposed side are called the front contacts and are usually designed to cover minimum area consistent with the ability to collect the current produced by the solar cell. Geometrically, the front contacts usually comprise a series of interconnected parallel narrow bars. Typically, their width is in the 30--50 micron range; their spacing is in the 250--2000 micron range. The back contacts can be a simple continuous sheet covering the entire back surface, although some solar cell manufacturers employ gridded rear contacts in order to minimize the amount of metal used, thereby reducing cost.
Historically, silicon solar cells are made first with plated contacts. The system used was the deposition of nickel by electroless methods, followed by solder build-up to provide adequate conductance, and to provide an interconnectable (solderable) contact area. Later, the ohmic contacts for cells intended for artificial earth satellite power supplies were modified to improve adhesion and ohmic contact quality by first adding a thin layer of gold, deposited by electroless methods, prior to nickel deposition. As space requirements became more demanding (minimum pull strength requirements increased; there was a need to contact a highly polished surface of silicon; the junctions became more shallow and adaptability to welding became a requirement), the contact structure was changed to an evaporated two-layer structure. The first layer consisted of titanium, the second of silver. Later to increase moisture resistance, a thin layer of palladium was added between the titanium and silver layers. This contact system has proven to be satisfactory for space cells and has been adapted to masking methods capable of providing the closely spaced narrow grid pattern required for front contacts on space cells and on cells for concentrator systems.
In addition to these evaporated or plated metal contact systems, various forms of metal pastes and inks, deposited mainly through silk screen masks, have been used for ohmic contacts by some manufacturers. In a few cases, the paste has been dispersed in photo-resistive matrices and patterned by photoexposure methods. These methods are still in a development stage.
Another approach involves the development of a solder-palladium-nickel contact system in which palladium is deposited in thin layers by an electroless plating method and the palladium layer is annealed at low temperature (approximately 300.degree. C.) to form Pd.sub.2 Si. This layer forms the ohmic contact base and has good adhesion to the silicon. Nickel is plated by the electroless method over this Pd.sub.2 Si layer and the contact is finished by a solder build-up. In this system Pd.sub.2 Si acts as a diffusion barrier for nickel, which can deteriorate the quality of the charge-separating p/n junction after diffusion during the high temperature (400.degree. C.) heating cycle used to sinter the contacts.
All of the contact systems described above employ metals thick enough (20 to 100 .mu.m) to ensure sufficient conductance. In addition to the resistance of these metal strips, the "contact resistance" between the silicon wafer or base and the contact metal must be minimized. This contact resistance is determined by two factors: the doping level of the silicon surface and the detailed electronic energy band properties of the metal which determine the resulting contact potential. Even if the contacts satisfy the criteria based on these two factors, the contacts must adhere firmly to the semiconductor base. This is necessary to ensure that the contacts do not deteriorate during use. Mechanical durability also is desirable to ensure ease of handling when arrays are being assembled and to withstand stresses in operation, for example, thermal cycling, etc. In addition, the contacts should be stable, i.e. should not corrode and the metals used in the contacts should not diffuse into the junction where they can reduce cell efficiency or even render the cell useless. Also, solderability and weldability of the contact metals is desired to simplify interconnections between cells.